So far, here's what I'm using and where in ThinkPad land:
T42 - 2GB RAM, equipped with OWC 60GB Mercury Legacy Pro SSD 2.5" IDE/ATA 9.5mm Solid State Drive, running Microsoft Windows 7 Ultimate, x86
T43p - 2GB RAM, equipped with Samsung 128GB 840 Pro 2.5" SSD, running Microsoft Windows 7 Ultimate, x86 (NOTE: required SATA mod to bypass original PATA bridge on internal drive bay)
T61p - 8GB RAM, equipped with Corsair Force Series F120 2.5" SSD, running Microsoft Windows 7 Ultimate, x64
U410 - 16GB RAM, equipped with Samsung 120GB 840 EVO mSATA SSD, running Microsoft Windows 8.1, x64 (IdeaPad, not a ThinkPad)
W510 - 32GB RAM, equipped with Intel SSD 520 Series 240GB 2.5" SSD, running Microsoft Windows 8.1 Pro, x64 and Crucial 512GB m4 2.5" SSD (data storage)
X120e - 16GB RAM, equipped with Corsair Force Series F120 2.5" SSD, running Microsoft Windows 8.1 Pro, x64
X220 - 16GB RAM, equipped with MyDigitalSSD 240GB BP4 mSATA SSD running Microsoft Windows 8.1 Pro, x64 and Samsung 512GB 840 2.5" SSD (data storage)
S230u - 4GB RAM, equipped with MyDigitalSSD 120GB BP3 mSATA SSD, running Microsoft Windows 8.1, x64 and Samsung 256GB 830 PRO 2.5" SSD (data storage)
So far, everything has been working very, very well. I don't actually have any information on any of the modern (U410 and later) systems performance on HDD vs. SSD, simply because I purchased SSDs (NewEgg, Amazon, Frys, or MyDigitalStore) and recovery media from Lenovo and did clean installs each time to the SSD when the machines arrived. None of them ever powered up with a HDD in them, except the W510, which had a 1TB HDD in the Ultrabay originally for storage.
System boot is generally in the 10-13 second range (measured until prompted for logon credentials).
I do not use any fast shutdown/resume/hibernation technology on any of the systems as hibernation is explicitly disabled via "powercfg -h off" command at an elevated Command Prompt. I'm more interested in the disk space than getting "boot" time down to 7 seconds (anything below 30 seconds if fine as far as I'm concerned), especially since all systems are equipped with the maximum amount of RAM, and I don't see any reason to eat 8/16/32 GB of SSD space.
What I have noticed is that there's no noticeable "hotspot" in the areas where the SSDs are located, and the machines are much more silent, with the cooling fans kicking in as needed.
The X220 has gone all day at a conference without needing to be plugged in. I haven't done that with the others, as I usually park myself near an outlet.
Performance remains excellent; I have not had any issues with speed degradation due to fragmentation, DRAM failure, or TRIM issues, even on the T42, which is using a PATA SSD (but TRIM is showing up as enabled, weirdly enough).
As far as SSD maintenance goes, I really haven't done anything, other than to keep their respective firmwares' updated. One of the MyDigitalSSD firmware updates was destructive (wiped drive) because it changed the spare space provisioning (I don't remember which one, off the top of my head)
As Astra.Xtreme noted, you don't necessarily go the mSATA route for speed (although you might with M.2), but for performance and power improvements over using a HDD. And, of course, they are still much faster than rotating media, just not as fast as their 2.5" brethren, usually in the write operations space.